Fabrication of optical devices with semiconductor fabrication technology and lithographic definition of photonic devices allows mass-manufacturing of optical devices and systems, enabling low cost production of compact, integrated solutions. A major difficulty remains however with coupling light on and off from optical chips. Indeed, the typical dimensions of on-chip single mode high index contrast integrated waveguides are on the order of a hundred, or a few hundreds of nm, while the typical dimensions of fiber optic cores are on the order of 10 μm (for single mode fiber) to a few tens of μm. This creates a dual problem of having to focus light from an optical fiber down to the dimensions of on chip high-index contrast waveguides and of having to precisely align the position of the optical fiber relative to the semiconductor chip.
A similar problem arises when directly coupling a semiconductor laser chip containing a laser diode to a primary photonic chip containing other optical devices such as single mode waveguides or modulators. While the dimensions of a semiconductor laser beam, such as is generated by a typical DFB laser or a typical Fabry-Perot laser, is typically much smaller than that of an optical fiber and more closely matched to the dimensions of high index contrast waveguides on the primary photonic chip, alignment constraints are even more stringent and need to be in the μm or sub-micron range. The typical beam dimensions of a semiconductor laser are on the order of 1 μm in the vertical direction, the dimension perpendicular to the laser chip surface, and on the order of 1 to a few tens of microns in the in-plane dimension.
Several methods have been demonstrated to couple light to and from a primary photonic chip with micron sized or submicron optical waveguides. One method consist in coupling to a waveguide from the top of the photonic chip, for example by using grating couplers as is taught in U.S. Pat. Nos. 7,068,887 and 7,260,289. This method has the advantage of relaxing required alignment tolerances since the waveguides on the primary photonic chip are tapered and broadened inside the grating coupler, or prior to routing to the grating coupler. This way a much larger beam is produced that is easier to align to. However, such grating couplers also induce other constraints. For one, such coupling schemes often only work for one polarization, or when they work for two polarizations they create additional complications such as the necessity of two photonic chip waveguides with uneven coupling from the fiber to the two waveguides depending on the state of polarization of the light. These methods are also poorly suited for direct coupling from a laser diode to a primary photonic chip, since the dimensions of the laser beam are significantly smaller than the grating coupler dimensions in at least one dimension, as the typical dimension of the laser beam in the direction perpendicular to the laser diode chip surface is on the order of 1 μm and typical grating coupler dimensions are of at least several microns. Finally, grating couplers are finite bandwidth devices that require tight control of the laser linewidth.
Other methods have been taught that rely on coupling light from and to the primary photonic chip by coupling light from above the photonic chip into the photonic chip through the surface of the photonic chip. An alternative to grating couplers is taught in U.S. Pat. No. 7,308,166. However, constraints relative to uneven coupling depending on polarization and stringent alignment tolerances remain. Couplers are either matched to beams that are much larger than the 1 μm typical dimension of a laser diode beam or alignment tolerance have to be submicron in at least one dimension in order to obtain high coupling coefficients.
Constraints relating to stringent alignment tolerances are difficult to satisfy and lead to high manufacturing costs not only because the alignment has to be established during assembly, but also because alignment has to be maintained after assembly, thus requiring very stable and reliable optical packaging solutions and leading to yield fallout.
An alternate method to couple light to and from a photonic chip consists in butt-coupling. Butt-coupling consists in routing a photonic waveguide all the way to the edge of the chip or to create a waveguide edge by other means, hereafter called the chip interface. Light can then be focused onto the waveguide cross-section at the chip interface of the photonic chip or light can then be collected from the waveguide cross-section at the chip interface. This method is generally more robust to polarization diversity since light is directly coupled without an interposed polarization sensitive coupling device. However, this method suffers from the fact that alignment tolerances are directly determined by the dimensions of the waveguide cross-section at the chip interface and are typically sub-micron for high index contrast waveguides. Also, assembly can be complicated due to the fact that the edge of the chip offers very little area to permanently attach a fiber or a laser, as opposed to the chip surface in the prior methods that allow permanently gluing a fiber, fiber array, laser diode, laser submount or optical bench to the chip surface. For this reason, it is typically required with edge coupling to position and attach both the primary photonic chip and the second optical element onto a common substrate, e.g. the optical package or an optical submount, in such a way such that the alignment between the primary photonic chip and the second optical element is maintained. This requires stringent control of the dimensions of the primary photonic chip, the second optical element and the common substrate as well as of the alignment tolerances between these elements.
A typical example of butt-coupling is the permanent coupling between a semiconductor laser diode and a tapered fiber in a butterfly package. This is a very costly packaging technology. In particular, the fiber is held in place by a metallic clip. The position of the fiber relative to the laser is adjusted by laser hammering, a process in which the metallic clip is repeatedly adjusted by subjecting it to a high power laser beam that thermally distorts the metallic clip.
In order to relax required alignment accuracies, it is common practice to taper the waveguide at the edge of the chip, that is, to progressively widen the waveguide while it approaches the chip interface. While it is straightforward to widen the waveguide in the direction along the chip surface, by simply drawing a wider lithographically defined waveguide, it is much more difficult to taper a waveguide in the vertical direction perpendicular to the photonic chip surface since in the latter case the dimensions are determined by the dimensions of deposited thin film layers. Thin films typically used in semiconductor chips, such as silicon, poly-silicon or silicon oxi-nitrides in the case of silicon based photonics are both difficult to fabricate with slanted cross-sections for tapering in the vertical direction and to deposit in thick enough layers to match an optical fiber cross-section. For both these reasons many implementations of tapered waveguides in the vertical direction rely on organic materials such as for example SU8. This is taught in “Fiber-Core-Matched Three-Dimensional Adiabatic Tapered Couplers for Integrated Photonic Devices” by Chun-Wei Liao et al., IEEE Journal of Lightwave Technology, Vol. 29, Nb. 5, page 770, Mar. 1, 2011.
Another form of taper is an inverse taper in which the waveguide cross-section of a dielectric waveguide is reduced while it approaches the chip interface, down to a cross-section well below the dimensions at which the waveguide first becomes single mode. The waveguide mode is poorly confined by a dielectric waveguide with such a small cross-section and expands again. This form of taper has the advantage that tapering in the horizontal direction can lead to both increased vertical and horizontal mode dimensions. The primary difference between a regular, i.e., non-inversed taper and an inversed taper is that in a regular taper the waveguide cross-section is increased, typically larger than a maximum waveguide cross-section such that the waveguide remains single mode, in order to expand the mode profile, while in an inverse taper the waveguide cross-section is decreased, typically smaller than a maximum waveguide cross-section such that the waveguide remains single mode, in order to also expand the mode profile.
State-of-the-art packaging methods with submicron or micron sized alignment accuracies typically rely on active alignment, a method in which light is coupled between the waveguide of the primary photonic chip and the second optical element during alignment and/or during attachment in order to monitor the quality of the alignment in real time. The quality of the alignment as given by the optical coupling efficiency is then used as feedback information in order to adjust the alignment. This method can result in technical constraints. For example, when aligning a photonic chip to a semiconductor laser diode, the semiconductor laser diode has to be operated during attachment. This complicates manipulation of the laser diode as it has to be electrically contacted and it constrains the alignment process as the diode has to remain cold enough in order to be operated. These constraints are taught in U.S. Pat. Nos. 6,559,464 and 6,970,628.
Chip placement accuracy in automatic pick and place systems can be as good as ±1.5 μm without active optical alignment in state-of-the-art commercial systems relying primarily on machine vision. However, this tolerance remains too high to passively butt-couple a typical semiconductor laser to a high index contrast waveguide. A typical off-the-shelf single mode laser diode as used in typical telecom or datacom systems has a horizontal beam width, i.e., a beam width along the surface of the chip, that is also on the order of one micron to at most a few micrometers. In this case, a misalignment by 1 or a few microns in the horizontal direction due to the tolerance of a passively aligned pick-and-place system very adversely effects the coupling efficiency between the laser diode and a waveguide located on the primary photonic chip.
Alignment accuracies can be relaxed by tapering a single mode waveguide on the primary photonic chip to a wider cross-section in the horizontal direction while approaching the photonic chip interface, as explained above. Tapering the horizontal dimension to a much wider width than the width of the laser beam significantly relaxes the required alignment tolerance, but also reduces the coupling efficiency obtained under optimum coupling conditions if the laser beam width is not increased accordingly, hence there is a trade-off between the peak coupling efficiency and the required alignment tolerance. This is caused by the fact that widening of the waveguide without widening of the laser beam results in a mode overlap mismatch that reduces the coupling efficiency in a single mode system, i.e., in a system where the coupled to waveguide on the primary photonic chip is single mode for at least a portion of its path.
The trade-off between the required alignment tolerance and the peak coupling efficiency can be relaxed by also widening the width of the laser beam, since a good mode overlap is then recovered. In principle, widening the width of the laser beam can be easily obtained in the horizontal direction by defining a wider laser strip on the laser diode chip, since dimensions in the horizontal direction are easily controlled by lithographic definition. This approach is however limited both by technical and economic considerations. Widening of the laser strip can lead to filamentation, a mechanism by which a semiconductor laser loses its single mode behavior in the spatial domain. This is taught in “High-Power Angled Broad-Area 1.3-μm Laser Diodes with good Beam Quality” by Chih-Hung Tsai et al., IEEE Photonics Technology Letters, Vol. 16, Nb. 11, page 2412, November 2004. Filamentation can result in a complex laser beam profile that can also change over time, both of which prevent efficient coupling of the laser beam into single mode photonic waveguides on the primary photonic chip. Widening the laser strip in conventional technology can also be difficult since it can make it harder to efficiently and homogenously electrically pump the laser beam.
Other methods to increase the width of the laser beam, such as tapering the laser strip close the edge of the laser chip typically results in increased manufacturing cost. Since the optical gain material on the laser diode chip results in high optical losses when it is not pumped, and since pumping it efficiently in the broadened laser strip region is both technically challenging and leads to excess current consumption, it is typically necessary to selectively remove the laser gain material in the tapered region and to selectively regrow another material in its stead. This is a very expensive and typically poorly yielding process.
Moreover, conventional off-the-shelf telecom grade laser diodes are often meant to be coupled to a tapered fiber. In this case, it is very desirable for the laser mode to have a circular beam pattern, since the tapered fiber also has a circular beam pattern and needs to be mode-matched to the laser. For this reason, off-the-shelf semiconductor lasers are often optimized to have a vertical beam dimension that is similar to the horizontal beam dimension. While it is possible to manufacture lasers with horizontal beam sizes specifically tailored for another coupling scheme, it is more cost efficient for the primary photonic chip to be compatible with preexisting off-the-shelf laser diodes.
In addition to the horizontal alignment accuracy, the problem of vertical alignment accuracy also has to be addressed. One possibility is to flip and attach the semiconductor laser chip onto the top of the primary photonic chip, but to first partially etch into the primary photonic chip so as to create a waveguide edge by etching through the waveguide and so that the laser beam is vertically aligned with the waveguide edge of the primary photonic chip. The laser is attached to the primary photonic chip in the etched through region, so that the relative alignment is controlled by the depth of the etch. This is taught in “Hybrid Integration of InP Lasers with SOI Waveguides Using Thermocompression Bonding” by M. Kapulainen et al., Proceedings of the 5th IEEE International Conference on Group IV Photonics, pages 61-63, 17-19 Sep. 2008. With butt-coupling the chip interface is often the edge of the chip. However in this variant of butt-coupling the interface of the primary photonic chip is not the edge of the chip itself, but the edge defined by the etch. The top of the chips is the side of the chips on which the relevant photonic devices are fabricated.
In this method, the vertical alignment between the laser and the waveguide on the primary photonic chip is determined by thin film layer thicknesses on the primary photonic chip and on the laser chip, by the etch depth into the primary photonic chip and by the attachment process used to attach the laser chip to the primary photonic chip. Thus, in principle, the vertical alignment between the laser beam and the waveguide on the primary photonic chip can be controlled with a high level of accuracy.
In practice, however, the vertical alignment accuracy is limited by the attachment process. Some alignment processes such as covalent bonding or anodic bonding result in a very high vertical attachment accuracy since the two chips are directly attached to each other without interposing an additional adhesive or bonding layer. However, they also require ultra planar surfaces on both the laser chip and on the photonic chip. Off-the-shelf laser diodes typically have a non-flattop chip surface due to the definition of the laser strip (or other forms of laser cavities) and to the definition of top electrodes, the top surface of the laser chip being the chip side on which the laser cavity is defined. Anodic or covalent bonding also require extremely clean chip surfaces and cleanroom environments with very low particle counts. With these bonding methods electrical contacting between the top surface of the laser chip and the top surface of the photonic chip is also complex since the adhesion does not occur via a conductive layer. Other methods such as eutectic or thermo-compressive metallic bonding of the laser diode require less planarity, are more particle tolerant and facilitate electrical contacting of the laser diode. They result however in degraded vertical alignment accuracy on the order of a few 100 nm, since the bonding layer can get more or less compressed. At the extreme, controlled collapse bump bonding is very tolerant to high particle count (dust) and non-planar topographies, but results in very poor vertical alignment control, with tolerances of at least one to several micrometers.
As with horizontal alignment tolerances, vertical alignment tolerances can be relaxed if the waveguides in the photonic chip are larger in the vertical direction. As with horizontal alignment tolerances, this alignment tolerance relaxation method is limited by the fact that a vertical waveguide dimension substantially larger than the vertical laser beam dimension leads to reduced overlap and reduced peak coupling efficiency.
The difficulty in coupling to a single mode waveguide located on a primary photonic chip from an incoming input light beam external to the primary photonic chip results from the reciprocity principle. The coupling efficiency is identical to the overlap of the time-reversed version of the input beam with the uniquely defined beam created by shining light out of the single mode waveguide. A misalignment of the input beam on the same order than the full width at half maximum of the input beam will thus lead to a substantial reduction of coupling efficiency if the input beam is well matched to the waveguide mode. On the other hand, in the case of a multi-mode waveguide the coupling efficiency from the input beam to the waveguide is equal to the overlap between the time reversed version of the input beam and the best possible linear superposition of beams created by shining light out of the waveguide for each of the modes supported by the waveguide. In such a case, the waveguide cross-section can be made much bigger than the input beam without penalty in maximum coupling efficiency, since the mismatch is compensated by the degrees of freedom afforded by the multi-mode nature of the waveguide. The degrees of freedom afforded by the multi-mode nature of the waveguide also allow maintaining a high coupling efficiency even for substantial displacements of the input beam. In general, in order to maintain high coupling efficiency from an optical element A to an optical element B, the light carried in optical element B needs to have as many degrees of freedom as the light carried by optical element A, including those resulting from displacing the relative position of optical element A.